What is an Auxiliary Power Unit?

Suparn Verma in Bombay

Auxiliary Power Unit is an engine normally placed near the tail of an aircraft to provide air-conditioning on the ground, thereby keeping it independent of any ground support. It is also used to provide air pressure to start the engines. A jet engine, unlike automobile engines, must rotate at approximately 20% of its top speed before fuel is injected and ignition occurs.

If the APU fails, the aircraft has no electrical supply, so it will be in total darkness. If the airport authorities at Kandahar have an electrical power source available -- for the Airbus 300 -- it can replace this APU. With the air conditioning not operating and with prevailing sub-zero temperatures it must be freezing in the aircraft. The only way to alleviate the discomfort of the people on board is if the ground authorities have an air-conditioning unit to heat up the aircraft, which in our opinion will not be available for such a large aircraft in Kandahar.

The conditions in the aircraft can have a positive effect on the passengers...in the sense that they will be too occupied keeping themselves warm to give in to panic too much. For the hijackers, however, the effort to keep themselves warm will probably affect their ability to keep their wits about them.

The APU could have failed due to mechanical reasons (the A300 APU is known to be unstable and shuts down very often due to the protections fitted on this APU). However, there is also the possibility that it was shut down by the crew which involves just the flick of one switch to the 'off' position. This is something that the hijackers would not even notice because even after switching it off, it can be moved to the 'on' position, but it will not start the APU unless another switch (the start switch) has been activated. Could this have been done to prevent the aircraft from taking off again?

A anti-hijack unit in Delhi regularly trains on aircraft parked on the tarmac at night. In spite of the all doors being closed, the commandos can storm the aircraft in less than a minute. The only possible points of entry are the main doors. An Airbus would have 8 doors. Possible points of
approach to these doors would be from the tail, or to walk on top of the plane (on the fuselage) and then descend towards the doors.

An explosion on board would be disastrous if it takes place in the middle section of the aircraft due to the presence of fuel tanks in the wings and in the belly of the aircraft.